You say "new" as in brand new latest production run or just a new older rifle? I had an old one in the late 70's [Micro groove] which shot very well even with cast bullets 250gr Keith to be exact.My newer one which has a Ballard barrel shoots lights out with anything I've run through it so far.Try a Hornady 265gr SP and 45 grs of 4198./.just can't believe this won't work for you.Maybe bench shooting would be a tad better and more accurate than out the window of the truck..you might try that.
